This invention relates to Schottky diodes and methods for their fabrication, and in particular to a Schottky diode having improved reverse bias characteristics.
A Schottky diode is fabricated by placing metal in contact with a light-to-moderately doped n-type semiconductor material. The resulting metal-semiconductor junction operates like a diode, conducting current in one direction (from the metal anode to the semiconductor cathode) and functioning as an open-circuit in the opposite direction.
Schottky diodes are used widely in electronic systems such as amplifiers, receivers, control and guidance systems, power and signal monitors, and as rectifiers and clamps in RF circuits. Commercial applications include radiation detectors, imaging devices, and wired and wireless communications products. High frequency Schottky diodes may be GaAs devices, and frequently are discrete devices.
The voltage-current characteristics of Schottky diodes are very similar to the voltage-current characteristics of p-n junction diodes, with two exceptions. Firstly, in a Schottky diode forward current is conducted by majority carriers (electrons). Under moderate forward bias conditions, minority carriers have a negligible role in operation of a Schottky diode. Thus Schottky diodes do not exhibit minority-carrier charge-storage effects found in forward biased p-n junctions and which gives rise to the diffusion capacitance of a p-n function diode. As a result, Schottky diodes can be switched from one state to another, either from on to off or from off to on much faster that p-n junction diodes.
Secondly, the forward voltage drop of a conducting Schottky diode is lower than the forward voltage drop of a p-n junction diode fabricated in the same semiconductor material for a given current. For example, a Schottky diode fabricated in silicon exhibits a forward voltage drop in the range of 0.3 to 0.5 volts, compared to a silicon p-n junction diode which exhibits a forward voltage drop in the range of 0.6 to 0.8 volts. When manufactured in gallium arsenide, Schottky diodes exhibit a forward voltage drop of about 0.7 volts.
Unguarded Schottky diodes typically have poor reverse leakage and poor breakdown characteristics. To improve leakage characteristics, high performance Schottky diodes are provided with junction guard rings. Guard rings provide excellent breakdown characteristics in both forward and reverse bias. Conventional junction guarded Schottky diode structures are fabricated by implanting a ring-shaped p-n junction in the semiconductor, typically silicon, forming an oxide surface layer by oxide growth and or deposition, opening a window in the oxide layer, and blanket depositing the Schottky barrier metal. Variations on this method have been proposed, but typically they create the guard ring prior to forming the Schottky metal contact. The guard ring introduces minority-carrier charge storage resulting in a slower switching of the Schottky diode. See for example U.S. Pat. Nos. 3,694,719 and 4,607,270.
A gated guard ring structure for a Schottky diode is disclosed in copending U.S. patent application Ser. No. 09/551,050 filed Apr. 18, 2000, entitled Self Aligned Gated Schottky Diode Guard Ring structures, the disclosure of which is hereby incorporated by reference. The gated guard ring structure provides an effective means of fabricating the gated Schottky diode using a self-aligned gate process. The portion of the substrate under the metal oxide semiconductor gate inverts when a bias is applied to the anode of the Schottky diode. Inversion of the substrate effectively forms an extension of the guard ring at low reverse bias so that the depletion layer between the p-n junction guard ring and the Schottky junction forms before sharp edge breakdown of the Schottky junction. Under forward bias conditions, the substrate under the metal oxide semiconductor gate is biased into accumulation, effectively removing the influence of the guard ring. While the resulting Schottky junction provides improved operation, a trade-off is made in that there are several additional steps to fabricate the Schottky diode.
U.S. Pat. No. 3,694,719 teaches a Schottky diode having metal deposited over an insulating material that has two thicknesses. The insulating material is stepped from an outer region away from the metal-semiconductor junction that is thicker to an inner region surrounding the metal-semiconductor Schottky junction that is thinner. Two methods are taught to achieve stepped thicknesses of the insulating material. The first method teaches a separate etch step to etch the thick layer of insulating material down to the desired thickness of the thinner inner region.
The second method teaches completely removing the portion of the insulating layer over the area of the semiconductor body surface within the guard ring to expose an area of the semiconductor body surface. Subsequently, the exposed area of the semiconductor body surface is recoated to the desired thickness with a thinner layer of the insulating material. An opening to the semiconductor body surface is then formed in the thinner layer of the insulating material, leaving a perimeter of the thinner layer surface of insulating material.
What is needed is a Schottky diode and method of fabricating a Schottky diode that retains the improved operation of known gated Schottky diodes but can be fabricated in fewer processing steps.
In accordance with the invention, a Schottky diode is fabricated by a sequence of fabrication steps. An active region of a semiconductor substrate is defined in which a Schottky diode is fabricated. At least first and second layers of insulating material are applied over the active area. A first layer of insulating material, having a first etching rate, is applied over the active area. A second layer of insulating material having a second, greater, etch rate is applied over the first layer of insulating material to a thickness that is about twice the thickness of the first layer of insulating material. The insulating material is patterned and a window is etched through the layers of insulating material to the semiconductor substrate. Metal is applied and unwanted metal is etched away leaving metal in the window forming a Schottky contact therein. One or more barrier layers may be employed.